12.7 Electromagnetic Induction

Ok, today we're going to continue with where we left off with motors. What we're going to do is the reverse. So let's just quickly review. When we had a loop of wire between magnets, if we had our current going through the loop of wire, we had our north and south magnets around this. Remember that we could use our left-hand rule to figure out what the force would be upwards on this layer, into the page, on that wire, so we'd get this motor to spin around. And then with motors, we just alternate the current, remember, so that every 180 degrees, the current flows in the opposite direction, and that keeps our force spinning around and around in a circle. So that was how a motor works. Now, what we're going to do is figure out the reverse. If we pass current through a wire in a magnetic field, that creates force. So scientists thought, well, will the reverse work if we force a wire... With no current through a magnetic field, would we then create current? There were several scientists who worked on this around the same time as Michael Faraday. Faraday was the first to publish his findings, but an American named Joseph Henry also discovered it independently. Faraday called it electromagnetic induction and the idea was that if we can create a magnetic field through a wire, then we should be able to create current. The experiment he used was quite simple. He suggested connecting a battery to a coil of wire wrapped around a metal ring, allowing the current to pass through easily. If we use the left-hand rule and suppose the current goes in a certain direction, we will observe a north and south pole based on the left-hand rule. Coil of wire. So, that's going to turn that coil into a magnet. With an iron ring, there would have a magnet on the other side. So, then his logic was, well, if this becomes magnetic on this side here, and it really doesn't matter which way it goes, so if the coil, the magnetic field turns the other coil into a magnet, then the logic is that should pass a current into the ammeter to be detected. So, what Faraday did is he set up his experiment like this: he closed the switch and thinking it's gonna work and he's gonna get current, but he noticed basically nothing happened when he closed the switch. The ammeter still showed zero. So then he thought maybe it was a mistake to unhook the battery, rehook it up again, and just by fluke he noticed that when he closed the switch on or off, ya meter blipped, but then it would go back to zero. So, just like you would do, he basically... Flicked the switch on and off, right through it very fast. And what happened then is he saw that he could actually create a current. So, the current would stay fairly consistent if he went really fast back and forth. So, his theory, Faraday's electromagnetic induction law, is basically: if we have a changing magnetic field, as long as the magnetic field is changing, then we're going to be able to create a current. So, using that logic, all he had to do really is instead of switching the switch back and forth, all you have to do is basically move the ring. If you spin the ring, that's going to change the magnetic field, then you should be able to get a current. And that's pretty much how a generator works. So, motor, we use electricity to make it spin. The generator, we get something to spin and that's going to create the electricity. So, if this iron ring is spinning around and around, then we get a constant supply of electricity. So, what we're going to do is instead of looking at something complicated like that, we're going to keep it pretty simple. We're going to look at just pushing a straight wire. Instead of having a ring, we'll have a wire. So, let's suppose we have a wire in this direction. And let's suppose we're going to push it to the right, through a magnetic field that's coming out of the page. Okay, so the idea then is if we push this wire in the magnetic field, we should be able to generate something. So, the challenge is, if you're going to push this thing, will the current flow? So, we use our left hand rule again, buddy. We've got to be really careful that we can't use it the same way we did before. Our B finger is still over the page, going out of the page. But, we've got to be careful because we're pushing this thing to the right. So, that's not our F. We don't put our F finger to the right. And then, our thumb, if we do that, it'll be going up, with F going to the right. So then, where our thumb would be pointing down, we think that our current is going down, which is wrong. So, what we have to do is make sure we're really careful with our. Fingers, so in this case, let me just kind of recap it. So, we're gonna push it to the right again, or B is still low to the page, so we want to keep our B finger out of the page. But instead of being a wire, think of it like being just one electron. We're actually forcing that electron to the right, so we put our B finger out of the page, our thumb to the right. Now, we get our force going upwards, and it's that force that's going to push that electron upwards. It's going to move the electron upwards, which then is going to create a current if there's a whole bunch of electrons. So it works the same, you just gotta remember that here F finger now is the direction of the way the current is going to go and your thumb is the whole wire being pushed one way or the other. That's really the only trick, is to get that straight. The rest of it works exactly the same. So for this example number one, at the bottom of the page, let's try to put your thumb downwards. We're pushing the layer. Downwards, our B is into the page. So, put your P down into the page. This means that our F would go to the left. Therefore, our current will flow to the left. And that's all.